arm: Add ENTRY/ENDPROC to private libgcc functions

When CONFIG_SYS_THUMB_BUILD is defined these functions may be called from
Thumb code. Add the required ENTRY and ENDPROC bracketing so that BLX is
used to call these ARM functions, instead of plain BL, which will fail.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Pavel Machek <pavel@denx.de>